The present invention relates to a zoom lens system which comprises four lens units, makes use of a rear focus and has a short total length, a large aperture and a high zoom ratio.
Recently radical reductions in the size, weight and cost of video cameras have caused the camcorder market to grow vigorously. The market growth has allowed widespread use of camcorders. Mainly, the video camera comprises an electric circuit board, an actuator (mechanical) system and an optical system. So far, the size and cost reductions have been achieved in terms of the electrical system in particular. More recently, however, some considerable reduction in the size of an image sensor optical system has made sharp progress. At present, the size and cost reductions of the image sensor optical system are being achieved by the development of a new zoom type lens systems making effective use of the progress in techniques for miniaturizing images, processing aspherical surfaces rotation-symmetrically and automatic TTL focusing. Several examples of those novel zoom lenses are set forth in JP-Kokai-62-24213, JP-Kokai-62-178917, JP-Kokai-62-215225, etc. However, the present need for size and weight reductions, esp., reductions in the total length and the diameter of front lenses, is immense.
Referring now to techniques prior to the above-mentioned prior art, for instance, JP-Kokai-60-186818 (a zoom lens of eight magnifications), the zoom lens system disclosed therein comprises a zoom subsystem constructed from a first unit having a positive refracting power, movable for focusing and consisting of, in order from the object side, a negative lens, a positive lens and a positive lens, a second unit having a negative refractive power, movable for zooming and consisting of, in order from the object side, a negative lens, a negative lens and a positive lens, and a third unit having a negative refractive power, disposed so as to correct variations of the position of an image due to zooming, the third lens being movable and consisting of a meniscus-form of negative lens component strongly concave on the object side; and an image formation subsystem constructed from a stop and a relay lens unit. This is an example of the zoom lens system having a long total length and a front lens with a large diameter. The first unit is designed to be movable for focusing, in which case it is likely that variations of spherical aberration may be increased by focusing in the vicinity of the telephoto end, thus making it impossible to increase the power strongly enough. Accordingly, the image location due to the first unit (i.e., the object point of the second unit) becomes so far that the power of the second unit can become weak and so much space is required for the movement of the second unit, resulting in an increase in the total length of the zoom subsystem. In addition, when the powers of the first and second units are weak, the location of the entrance pupil is imperatively deep (as viewed from the object side). In particular, the lens diameter of the first unit should not be increased; nor should the lens thickness be increased so as to assure the edge thickness of the convex lens. This in turn leads to an increase in the depth of the entrance pupil, resulting in a massive increase in the size of front lens and, hence, a further increase in the total length of the zoom subsystem. To add to this, consideration must be taken of a further increase in the depth of the entrance pupil when a near-by object point is brought into focus. This is because focusing is achieved by the first unit. For those reasons, such rear focus versions are set forth in the above-mentioned JP-Kokai-62-24213, 62-178917 and 62-215225 and some versions in which the compensator unit is located in the rear of a stop have now been adopted. These versions may possibly have astonishing latent faculties of reducing the total length and making the front lens diameter small. Referring especially to JP-Kokai-62-178917, it teaches that some considerable reduction in the number of constituent lenses is achieved by applying an aspherical surface to the above-mentioned image formation subsystem and some correction of aberrations is also attained. However, any size reduction cannot be achieved with the total length and front lens diameter without making substantial changes with respect to conventional or classical lens configurations.
That is to say, the system disclosed in JP-Kokai-62-178917 comprises a zoom subsystem including a first unit having a positive refracting power and a second unit having a negative refracting power and an image formation subsystem including a third unit consisting solely of a positive single lens having an aspherical surface and always remaining fixed and a fourth unit having at least one negative lens or consisting generally of two or three lenses and being movable during zooming and for regulating the focal position depending on how far the subject is spaced. Thus, the use of the rear focus which also serves as a compensator and the aspherical surface renders it possible to reduce the number of constituent lenses to ten or below, thereby reducing extra space. This thus enables the front lens diameter to be reduced and makes it possible to reduce the total length. The rear focus configuration could make it easy to increase the power of the first unit. However, this is not the case and the power of the second unit remains weak as well. In addition, using a single lens for the third unit incurs another disadvantage. In other words, the luminous flux cannot be easily converged here, so that it can not leave a focally. For this reason, there is no choice but to increase the focal length of the fourth unit, and the back focus remains long as well; that is, no sufficient reductions in the total length, front lens diameter, etc. are achieved.